1. Field of the Invention
The present invention relates to a control device for a primary braking system, more particularly, to a primary retarder in a vehicle, as well as to a method to control a primary braking system.
2. Description of the Related Art
Primary braking systems generally refer to braking systems which are positioned in front of the clutch and the transmission. (Ref. Kfz-Anzeiger. 44th year, 1991, report: "Safely Downhill", page 30 and Lastauto Omnibus April, 1991, report "Against the Current", page 30). Secondary braking systems, on the other hand, are mounted directly on the transmission or in the drivetrain downstream of the transmission, and act on the rear axles of the vehicle.
Next to primary retarders, there are the following additional primary braking systems that have become well known in the industry: Guillotine-type exhaust brake; bleeder brake; compression release brake
The guillotine-type exhaust brake relies on the increased backpressure to generate the braking torque. During braking, the exhaust piping is nearly fully blocked in order to impede the outflow of the cylinder charge during the exhaust stroke, thus elevating the braking power of the engine.
The bleeder brake (Ref. Kfz-Anzeiger. 44th year, 1991, report: "Safely Downhill", pages 10-13) is a type of brake which utilizes an additional valve, a so-called decompression valve which is integrated into the cylinder head. When activated during the third stroke of the cycle, it substantially lowers the expansion pressure acting on the piston and, thus, lowers the acceleration of the piston. As a result, a difference develops between the compression and expansion work, which can be used to increase the braking power of the engine. In particular, such a braking system can be modulated by controlling the throttle positions.
In a further development of the bleeder brake, the decompression valve is not kept open during the entire working cycle, but only during a short time span (ref. MTZ Motortechnische Zeitschrift 56 (1995) 7/8 pages 418-423; "The New Decompression Release Brake (DVB) from Mercedes Benz").
The control of such an engine brake can be performed by a proportional control valve, serving the function of a pressure control valve with the capability of affecting the opening characteristics of the decompression valve.
For further details, the above indicated article of the MTZ Motortechnische Zeitschrift 56 (1995) 7/8 page 422 is recommended as reference.
The compression release brake is, just like the bleeder brake, an engine brake. In the case of the compression release brake, a valve control device releases the compression, which leads to--as with the bleeder brake--a substantial increase in engine brake power (ref. Kfz-Anzeiger, 47th year, 1/1994, report: "Elegantly Packaged--Test Report Volvo FH", pages 10-12).
While the control device and the control method, in accordance to the present invention can be, as indicated above, applied to any primary brake systems, primary retarders also provide a unique application for this invention. It is, unlike the secondary retarder which is normally mounted between the transmissions and the propeller shaft, acting directly on the engine, as already described above. A primary retarder, which is permanently connected to the engine, is known from German Patent document DE 44 08 349, incorporated herein by reference. Due to the direct connection of the primary retarder to the engine, it is particularly important on a configuration such as this one to assure that the retarder is deactivated upon separation of the driveline from the transmission, or upon activating the clutch or shifting the vehicle into neutral, in order to keep the retarder from slowing down the engine. If this is not assured, a rapid load change, as a result of a clutching or de-clutching event, can lead to unacceptable low engine speeds because of the braking action of the retarder.
This problem becomes especially acute whenever the retarder is activated during braking operation, especially during the basic retarder functions "braking mode" or "V-constant". The basic function "braking mode" is characterized by a fixed brake setting of the retarder. This can be achieved by use of a hand brake lever of a foot pedal, which, based on a fixed correlation, achieves a certain braking torque, which can range from a minimum braking torque M.sub.min to a maximum braking torque M.sub.max.
While operating the brake at "V-constant", the retarder can be, from a controls standpoint, coupled with the cruise control. The retarder braking action is achieved by controlling the braking torque so that a constant speed can be achieved, i.e., during downhill operation.
Control devices or methods for the control of a retarder with respect to its braking torque have been published in U.S. Pat. No. 5,507,361, which is incorporated herein by reference.
In order to avoid a conflicting operation between the retarder and the engine, i.e., at elevated speeds, German Patent Document DE 43 41 213 proposes to establish a relative priority of the individual systems for specific applications. For example, it is proposed to always yield the priority to the retarder in case the throttle and retarder are activated simultaneously. If the "constant velocity" function of the retarder is activated, the application suggests moving the throttle lever of the engine to the idle position.
A conflicting operation between the retarder and the engine is, according to the controls logic described in German Patent Document DE 43 41 213, acceptable for short periods of time during shift events.